Seriously that is a HUGE myth.
You are more likely to become dehydrated in the harsh winter months than in the hot ones.
In the winter months, we think we don’t sweat as much as we do when outside when it is 90°. In actuality, we do sweat as much. It just evaporates much faster in the colder temperatures.
Typically in cold temperatures the dew point drops. This means there is significantly less water in the air. When you breathe in, you receive less water than when it is very humid.. This is also why you see your breath when you step outside. The water from your exhaled breath immediately vaporizes and becomes very tiny particles of water.
Unless you are a polar bear, most of us crank up the heat in our dens. This is a massive amplifier to drying out our air. It makes your external organ—your skin!— very dry. Cranking up a humidifier or even adding more plants to your living spaces are some simple tricks to adding water back into your home’s air.
The combination of quick evaporation, high dew points, plus our need for heat, quickly leads to dehydration. Drink that extra glass of water, finish your soup broth and try a few different teas at night instead of that soda!